In recent years, high wet grip performance and low rolling resistance have been demanded for pneumatic tires. To satisfy these, technologies in which a styrene-butadiene copolymer and a reinforcing filler such as silica are blended in a rubber composition constituting a cap tread of a tire have been known. Furthermore, to enhance wear resistance and rubber hardness of a rubber composition, for example, blending of polybutadiene and/or highly reactive silica has been proposed; however, in this case, problems of reduction in rubber strength, deterioration of processability, and the like exists.
Japanese Unexamined Patent Application Publication No. 03-239737A describes that a pneumatic tire manufactured using a rubber composition containing a styrene-butadiene copolymer having a particular arrangement of styrene-derived units, and silica in a tread can satisfy wet skid resistance, rolling resistance, and wear resistance at the same time. However, this rubber composition cannot always satisfy the demands from consumers since the rubber strength thereof decreases.
Japanese Unexamined Patent Application Publication No. 57-179212A describes a styrene-butadiene copolymer in which, relative to a total styrene content in a styrene-butadiene copolymer, a long chain styrene block is 5 wt. % or less and one single chain is 50 wt. % or greater in styrene-derived units, and a total content of styrene is 10 to 30 wt. % of the styrene-butadiene copolymer. However, this is not sufficient to enhance rubber strength, wear resistance, and low hysteresis loss of rubber compositions.